Mass utilization of petroleum has generated enormous pollution and obviously changed the global ecological environment. It is also the primary factor causing climate change and global warming. Facing severe pollution caused by petroleum, many nations and large businesses have paid attention to develop alternative energy, such as solar energy. As to solar energy, the energy of sunlight is converted into DC current or DC voltage via photoelectric conversion. DC voltage must be converted into high-voltage AC voltage before it can apply to common household appliances.
Refer to FIG. 1. A conventional DC-AC conversion circuit used in alternative energy system comprises a low-voltage energy input unit 1, a step-up converter 2, a DC-AC converter 3 and an output load 4. The low-voltage energy input unit 1 is a power source, such as a solar panel or fuel cell. The step-up converter 2 transforms the received DC voltage into a high DC voltage via amplification. The DC-AC converter 3 transforms the high DC voltage into an AC voltage. Then, the output load 4 outputs the high AC voltage. However, the above-mentioned two stages of voltage conversions greatly lower the reliability and efficiency. Referring to FIG. 2 for another conventional DC-AC conversion circuit, where the above-mentioned step-up converter 2 and DC-AC converter 3 are integrated into an integrated converter 5, whereby is reduced the complexity and area of the hardware circuit, wherefore is decreased the cost of the overall system. However, the amplification of the step-up converter 2 is still insufficient. Thus, the DC-AC conversion circuit can only apply to the cases having a higher-voltage DC power source.
U.S. Pat. No. 7,394,671 disclosed a DC-AC converter, which uses a chip to control the turn-on timings of two semiconductor switches to obtain a stable AC output. U.S. Pat. No. 7,626,834 disclosed a DC-AC converter, which comprises four power source switches, two diodes, insulation transformers, capacitors and etc., wherein charge and discharge of the capacitors are realized by the power source switches, and the electricity is transferred to the rear end and converted into AC output. In the above prior arts, the DC switches (the semiconductor switches or power source switches) have to resist higher DC voltage. If the switch does not have sufficient voltage resistance or current resistance, it may be damaged. As the DC switches have to resist higher voltage, they usually have higher price, larger volume, greater switching loss and insufficient step-up ratios.
U.S. Pat. No. 7,551,463 disclosed a current-limiting circuit to protect power source switches lest high voltage or high current damage the power source switches. The current-limiting circuit can effectively prevent DC switches from being damaged. However, the step-up ratio and conversion efficiency are still insufficient. Further, the current-limiting circuit occupies additional area. Therefore, the conventional DC-AC conversion circuits still have room to improve.